Coupled K+-CI- co-transport is mediated by the KCC proteins, encoded by four members of the SLC 12 cation-chloride cotransporter gene family. Genetics, physiology, and the characterization of knockout mice have implicated the KCCs in the pathogenesis of disorders as diverse as hypertension, epilepsy, renal tubular acidosis, neuropathic pain, and sickle cell anemia. In the renal proximal tubule, isotonic swelling induced by apical Na absorption activates basolateral K+ -Cl- cotransport mediated by KCC3 and KCC4, implicating these transporters in proximal re-absorption of filtered Na+-Cl- and other solutes. Indeed, in addition to reduced fluid transport, KCC3-deficient mice exhibit defects in the absorption of bicarbonate, suggesting that loss of KCC3 causes generalized proximal tubular defects. Furthermore, given the demonstrated role of KCC3 in regulatory volume decrease (RVD), the response to oxidant stress, and a severe neurodegenerative syndrome, we propose that KCC3 is required to maintain cellular integrity in response to ischemic volume increase ("IVI"), such that loss of KCC3 predisposes to ischemic tubular necrosis. The role and regulation of K+-Cl - cotransport in the proximal tubule is thus the focus of this competing renewal. We propose in Aim 1 to finish characterizing the renal phenotype of our existing mouse strain with germline deletion of the KCC3 (Slc12a6) gene, in addition to creating and characterizing a mouse strain with KCC3 deletion that is limited to the renal proximal tubule. These animal studies will encompass immunohistochemistry, renal physiology, and assessment of the response to renal ischemia/reperfusion injury. Whereas neuronal-specific KCC2 is unique in mediating constitutive K+-Cl - cotransport, the other three KCCs are quiescent in the absence of cell swelling. Using a chimeric approach, we have localized the molecular determinants of constitutive isotonic activity to a KCC2-specific expansion in the C-terminal cytoplasmic domain. Given the role of swelling-activated K +-Cl- cotransport in proximal tubular salt and solute transport we will characterize the molecular determinants of swelling activation, using chimeras between K CC4 and t he s welling-inhibited N a-K-2Cl cotransporter N KCC2. T he effect o f c ell volume o n phosphorylation status and membrane trafficking will also be studied in Aim 2, using KCC4 and several N-terminal variants of KCC3. These studies will begin in Xenopus oocytes but will ultimately be extended to the opossum kidney (OK) cell line and/or other epithelial cell lines. In Aim 3 we will focus on the role of conserved cysteines in the mechanism and regulation of K+-Cl - cotransport, building on data from both activating and inactivating mutations of conserved transmembrane cysteines. Various cysteine-depleted mutants will also be used to distinguish the cytoplasmic and membrane-associated mechanisms in the activation and inactivation of the KCCs by nitric oxide (NO) and related cysteine-reactive compounds.